Fluoxetine (Prozac ) and other serotonin-selective reuptake blockers (SSRIs) are being increasingly used to treat mood disorders in children. In adults, fluoxetine increases postsynaptic 5HT2A receptor signalling. In contrast, our data reveal that when administered prior to maturation, fluoxetine 5HT2A receptor signal transduction, an effect that is opposite to that produced in adults. However, virtually no preclinical data exist regarding the immediate or long-term changes in 5HT systems due to prepubescent SSRI treatment. The long-term objective of this proposal is to understand the mechanisms and persistence of adaptive changes in 5HT receptor systems produced by pubescent exposure to SSRIs. Because the clinical effectiveness of SSRIs is associated with adaptive changes in 5HT signal transduction, our HYPOTHESIS is that (1) prepubescent fluaxetine treatment will produce different neuroadaptations in postsynaptic 5HT1A and 5HT2A signal transduction than produced by adult treatment, and (2) the effects of prepubescent SSRIs will persist into adulthood and consequently, alter the ability of 5HT systems to respond to subsequent SSRI administration during adulthood. Aim 1 will determine the dose-dependence of fluoxetine-induced changes in postsynaptic 5HT1A and 5HT2A receptors and receptor-mediated neuroendocrine responses, and will establish the treatment dose for subsequent studies. Aim 2 will determine the biochemical mechanisms responsible for fluoxetine-induced adaptation(s) in postsynaptic 5HT1A and 5HT2A receptor systems, by investigating changes in specific components of the signal transduction pathway between 5HT receptors and their respective second messenger enzymes. Aim 3 and Aim 4 will investigate the longer-term effects of prepubescent fluoxetine treatment on changes in postsynaptic 5HT signal transduction. Postsynaptic 5HT1A (aim 3) and 5HT2A (aim 4) receptor systems will be studied with respect to: (1) the persistence of prepubescent fluoxetine-induced 5HT adaptations into adulthood and (2) the regulation of 5HT receptor systems in response to subsequent adult fluoxetine administration following prepubescent exposure. These studies will provide important new information about the mechanisms underlying the immediate and long-term adaptive changes in brain 5HT signalling due to prepubescent fluoxetine treatment. These studies will also elucidate the status of serotonergic function in adults treated previously with SSRIs as juveniles in order to predict how these individuals will respond to subsequent antidepressant treatment as adults. This information is critical to the effective use of SSRIs in treating mood disorders in children and in treating adults treated previously with SSRIs as juveniles.